1. Field of the Invention
The present invention relates to a signal processing circuit for a rotation detector.
2. Description of the Related Art
A conventional signal processing circuit for a rotation detector is disclosed, for example, in JP-A-2007-170992 (corresponding to US 2007/0139036 A). The rotation detector includes two magnetic sensors. When a rotor having gear teeth rotates, the two magnetic sensors respectively output rotation detecting signals Sa and Sb having different phases. As shown in FIG. 13, the signal processing circuit determines that a rotational direction of the rotor is changed based on a change in a phase relationship between the rotation detecting signals Sa and Sb and generates a reverse signal REV. In addition, the signal processing circuit reads all edges of the rotation detecting signal Sa.
Then, the signal processing circuit generates a level-change prohibiting signal for prohibiting changes in a signal level that synchronize with a first rising edge and a first falling edge of the rotation detecting signal Sa after the change of the rotational direction. Based on the level-change prohibiting signal, the signal processing circuit masks the first pulse of the rotation detecting signal Sa after the change of the rotational direction and generates a first output signal OUT1. Based on the first output signal OUT1 and the reverse signal REV, the signal processing circuit generates a second output signal OUT2 that transitions between a high level (H) and a low level (L) while the rotor rotates in the normal direction and transitions between the high level (H) and a middle level (M) while the rotor rotates in the reverse direction.
However, when the first pulse of the rotation detecting signal Sa after the change of the rotational direction is masked, it may be difficult to detect a rotational motion of the rotor with accuracy based on the second output signal OUT2.
For example, in a case where the rotational direction of the rotor is changed with a short period, if the first pulse of the rotation detecting signal Sa after a change of the rotational direction is masked, the second output signal OUT2 does not change while the rotational direction of the rotor is successively changed. Thus, when the rotational direction of the rotor is successively changed, the signal processing circuit is difficult to detect the rotational motion of the rotor with accuracy.